Keyframe a driver / Triggering scripted expression with a keyframe
You can use Python's ternary if-else operator:

function_of(y_location) if frame &gt;= 32 else 0


You can nest these.

If you need to evaluate some object's position at given frame, you have to use PyDriver.

Like batFINGER suggested, binding this to the start and end position of the teeth rack (or to 2 empties) would work well also. That would look like this:

(rot_fce(y_loc) if y_loc &lt; end_loc else rot_fce(end_loc)) if y_loc &gt;= start_loc else 0